Pneumatic percussion apparatus such as pneumatic drills, picks, concrete or rock breakers and the like essentially comprise a cylinder within the interior of which a ram or hammer is axially reciprocated by a reciprocating valve controlling the feed of compressed air to the cylinder, the ram or hammer functioning as a piston which drives a tool such as a chisel, pick or drill.
This ram is subjected to linearly reciprocating movement under the pneumatic control, i.e. is driven downwardly by pneumatic pressure until at the end of its stroke it impacts against the upper end of the tool or an anvil affixed thereto.
The tool can be received in a guide bushing which itself is slidable in the cylinder and forms a guide not only for the tool but also for the lower end of the ram and positions the tool shank in axial alignment with the ram.
The air-distribution valving systems for such devices, which can alternately feed the compressed air to the opposite side of the ram, are well known in the art, likewise are provided on such devices.
It is customary to form the upper end of the cylinder with a pair of handles which can be gripped by the operator, one of which can be equipped with a trigger or actuating lever which operates a feed valve for admitting the compressed air to the distribution system to turn on and off the pneumatic reciprocating or hammer action which is automatic when the latter valve is open and is precluded when this valve is closed.
In the use of such tools, the reaction of the forces generated by the compressed air upon the cylinder on the one hand and the return of the shock wave generated by the impact of the ram against the pick on the other hand, especially when the pick is in contact with a hard material such as rock or concrete, imparts to the cylinder vibratory movements which are transmitted to the hand and to the user of a frequency and energy which may vary but high values can affect the blood circulation of the user and the joints of the extremities of an operator. These vibrations are readily transmitted to the hands of the operator and thus through the body of the individual gripping the handles.
In order to reduce the vibratory movement which is detrimental to the health of the operator it has already been proposed to decouple the movement of the cylinder from that of the handles by providing an assembly which is slidably mounted on the upper end of the unit and carries the handles. A cushioning or shock-damping chamber is provided between this assembly and the upper end of the apparatus (see, for example, French Pat. No. 2,136,469).
This shock-absorbing chamber is supplied with air in the earlier system with a feed arrangement that has been found to create difficulties. For example, when the passages feeding the shock-absorbing chamber have a significant flow cross section, the pressure drop makes the pneumatic hammer significantly less effective. When, however, smaller flow cross sections are provided to avoid a significant pressure drop, they are readily obstructed by contaminants which are practically unavoidably entrained with the compressed air.
Thus the conventional system has been found to suffer from a significant energy loss on the one hand or possible obstruction of the passage which feeds the shock absorber chamber on the other so that the apparatus has either been inefficient or incapable of sufficiently protecting the user.